RichardS
N/A
The question is "Was the line set by the sun" and the answer was given in post #5.
Richard
Logitude
- Thread starter Graham_Wright
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The question is "Was the line set by the sun" and the answer was given in post #5.
Richard
GHA
Well-Known Member
Hydrozoan
Well-Known Member
The choice was between Greenwich and Paris.
The conclusion was that while sailors were happy with having 'GMT' I don't think that there were many who were looking forward to having to live with 'PMT' all their life...
But your fear is narrowly Anglophone - in the French it would I think be exactly the reverse: TMP, Temps Moyen à Paris.macd
Well-Known Member
Just did a forum search and apparently not.
Then he's done a runner, so obviously nothing wrong with his traction.
(There definitely was an Articpilot until quite recently. Well, as definitely as my memory permits.)
Graham_Wright
Well-Known Member
What I was fishing for was - Once it had been decided that a global meridian was needed, was it plonked down in Greenwich at a declared longitude of zero at the sun's highest altitude and then clocks set to 1200? Or was it the other way round?
Thereafter LHA would give angular distance from this meridian.
macd
Well-Known Member
Wouldn't it have been plonked down in Greenwich at a declared longitude of zero and it then declared that when the sun was at its zenith on that meridian it was noon GMT?
Graham_Wright
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That's what I was looking for confirmation of.
AntarcticPilot
Well-Known Member
No, Greenwich Mean Time is the average time at the location of the brass strip. Mean because of differences between solar and sidereal time cause by the ellipticity of the Earth's orbit round the sun.
As I said, this is how it USED to be defined, and is still the way it is assumed to be for celestial navigation. But the definition these days is in terms of a model derived from a network of VLBI stations, primarily.
macd
Well-Known Member
What other rational alternative is there? The time sets itself (solar noon). Surely the only variable is the reference position?
TLouth7
Well-Known Member
I hope you now have it
Graham_Wright
Well-Known Member
[70521]
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All the primary meridian is a reference point. A bit like "Aries".
Graham_Wright
Well-Known Member
Yes - as I assumed.
I assumed the reason I could not find a zero difference between noon and the GHA was because either the original fix was inaccurate (inevitable eventually) or the earth is still wobbling (it is).
The nearest I found this year was April 16 which showed a 3' angular difference.
LittleSister
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Skylark
Well-Known Member
Never seen this before. Not sure that I fully understand it. Can you please add the missing X axis and explain its use/interpretation in relation to the Equation of Time? (and possibly to the latitude of the observer).
TLouth7
Well-Known Member
This is the analemma, it shows the difference in position of the Sun from the mean on a given day and time. The x-axis is deviation of the apparent longitude, equivalent to deviation of apparent time.
The standard graph for equation of time has the x-value of this plotted (on the y-axis) against time of year. This makes sense because latitude of the Sun is not relevant to time (longitude) calculations.
The standard graph for equation of time has the x-value of this plotted (on the y-axis) against time of year. This makes sense because latitude of the Sun is not relevant to time (longitude) calculations.
GHA
Well-Known Member
If you do a time lapse photo snapping once a day at UTC noon every day for a year it will look something like that
Skylark
Well-Known Member
Many thanks for taking the time to respond.
Still sufficiently intrigued, I’ve just looked at Bowditch and the Nautical Almanac. Bowditch doesn’t refer to the analemma per se but there a couple of sections on Equation of Time.
Looking at the Almanac, I can now see that the X axis is time/longitude. Early November would be 16 minutes of time (EoT is 11h44) and early Feb would be 14 minutes of time (EoT is 12h14). I don’t quite get the relevance of your second paragraph. I’ve just plotted the analemma x axis as a y axis with time of year (mid month) as its x axis. But doesn’t time of year equate to declination? I’m not following what you’re trying to tell me, sorry.
What is the purpose of these two graphs? For what are they used?
TLouth7
Well-Known Member
If you look at the Wikipedia article for equation of time you will see a graph of deviation between sundial time and mean solar time:
https://en.wikipedia.org/wiki/Equation_of_time
This graph is a bit like the analemma but rather than lat
vs long (x) it shows long against time of year (x). Note how the peak in this graph in Feb corresponds to the right-most point on the analemma.These are/were relevant to mariners because if you find local noon on a given day you need to correct by some amount (as per the graph) to get mean local time, to then compare against GMT*. If you don't care about the Sun's latitude then the EoT graph is more convenient (or more likely tables in an almanac).
*Whether you apply the correction to local time or GMT, or work in angles is not important to the fundamental relation.
Skylark
Well-Known Member
Thanks for the additional explanation, TLouth7, much appreciated.